TheRadioBoard

I am in the process of building this circuit and have it running.



I wonder if anyone else has built it or a similar circuit.

Am having an issue with peaking the RF input. Seems like I still get two signals. One is +455KHz the other is -455KHz referenced to the local oscillator freq.

Otherwise it seems pretty sensitive. Definitely needs a vernier on the tuning cap and I'll have to conjure up a BFO for SSB reception. First attempt covers 5 to 10 MHz. I didn't use the IF gain control; I used an LED as a 1.9V regulator/reference.

Rich

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Homebrew Radio ex-Silicon Valley

What are you using for an AL value for the T37-2 toroids? The CWS Bytemark website gives an AL value of 4 mH/N^2, but this number doesn't make any sense given the dimensions and the mu value. It appears that they made a typo and the AL value should be 4 nH/N^2 (nanoHenries, not milliHenries). This then agrees with the toroid dimensions and the mu value. Given this, the RF tank inductance would be 2.3 uH, and the LO tank inductance would be 1.94 uH.

It appears that the INB RF input is effectively grounded for RF because of the large 0.01 uF capacitor to ground. Therefore the RF tank will consist of the 24 turn secondary of the T37-2, and C1 paralleled with C2a. The 2.3uH is then in parallel with the capacitance C1+C2a (20 to 215 depending on the settings). This will give an RF range of 7157 kHz to 23456 kHz. This seems like an enormous range, and it doesn't tune down to 5 MHz.

On the local oscillator side, the inductance of 1.94uH paralleled with C2b having a range of 10 to 35 pF will give an LO range of 19335 kHz to 36172 kHz. Again, with these values I don't know how you would get it to tune down as low as 5 MHz, unless there's something odd about how the NE602 oscillator works.

Here's an interesting circuit. Lots of useful ideas. I have not built it. If anyone has an interest in giving it a try, I have a few TA7358AP's. I'd be happy to send a sample (free, of course until I run out).

http://translate.google.com/translate?h ... rmd%3Divns

Hope this link works. Let me know if it doesn't.

Here's the link in Spanish:

http://www.lw3ewz.com.ar/modules.php?na ... le&sid=367

Bruce Kizerian KK7ZZ

it also uses the ta7358 .... http://py2ohh.w2c.com.br/trx/jurity/jurity2.htm ... is very simple .

73.

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"Deus é tudo"

The first link using google translate takes you to the guy's homepage. You can get to the translated version from there by clicking on "List of Articles" on the left, going down to "Fuentes, Linear, Instrumental, etc.." and clicking on "HF Receiver with 1.5 Volt".

Bruce

The dual variable is actually a 365 pF.

Both the tuning and LO seem to be OK. On the low end, I can hear WWV at 5 MHz and likewise on the hi end, WWV at 10 MHz. Before assembling, I checked both inductors with a 200 pF cap in parallel to make sure they resonated within the desired band.

http://toroids.info/T37-2.php

With the above calculator, 22 turns should resonate just below 7 MHz with a 300 pF cap; this is what I wanted.

5 MHz. is still a lot of band. This thing really needs a vernier drive, or maybe just a smaller variable cap.

I will experiment with different input circuit ideas and post more info if I make any improvements.

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Homebrew Radio ex-Silicon Valley

Okay, that makes sense then.

When I first read your post, I was thinking it was a tracking problem, but re-reading it, I see that you're getting both +455 and -455 signals at the same time and therefore it's definitely an image problem. So, your front end isn't selective enough. A higher frequency IF may help.

What a difference darkness makes!


I turned on my little receiver board last night and heard WWV at 5 and 10 MHz, plus lots of commercial stations in that range.

So, now to package it up for stability; maybe add a BFO and an RF stage.

I made my pc board using Express PCB software and would be glad to share the artwork with anyone who wants it. It's not a professional design and has no silkscreen pattern for component ID, but "it works!"

Rich

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Homebrew Radio ex-Silicon Valley

I've made lots of progress and improvements. Changing the variable cap on the LO was the biggest gain. The little Miller 365 mmf that I started with was too much capacitance. Too hard to tune in any signal except really powerful AM. I changed to a 100 mmf. Hammarlund with good bearings. Now I get nice smooth tuning. I think the original variable cap had a bad contact on the rotor wiper.

Added a BFO and varicap "fine tuning" so now I get lots of stations on 40 mtr. SSB.

Next version will have one more stage of audio gain ahead of the LM386; maybe an LM741.

All-in-all, it's been a good learning experience and am amazed at the number of stations I can get with a 25ft. indoor antenna.

Rich

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Homebrew Radio ex-Silicon Valley

One more change/improvement.

The original circuit uses a floating variable capacitor for RF peaking. That's a mechanical pain, though I was successful using APC-type variables. They have isolated mounting screws available.

In the revision, I simply connected pins 1 and 2 of the NE602 to the rest of the circuit using 0.01 uF caps. Now the inputs can never be DC-grounded.

Rich

PS: Also found out that my TEK LCD scope throws out all kinds of trash in the HF bands.

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Homebrew Radio ex-Silicon Valley

Hi Rich,

There's some *wrong* things you can apply to squeeze more gain out of the 386. In my Kitchin receiver I was left wanting a bit more audio gain and tried some of the tricks. JF1OZL (I think it is) squeezes something like 72db gain out of the 386 but it really sounds rather nasty with distortion at that level. I forget exactly where I settled with mine (50-60db?) but it sounds good and more than compensated for an additional gain stage.

Up-date.

My little radio has been re-built and modified several times. Now working well on 40 mtrs. I can cover the whole band with 180 degrees on a vernier dial.

Replaced the toroids with 10.7 MHz. IF cans from Mouser. Crushed the internal capacitors.

Noticed that sensitivity improves when I turn on the BFO. This must change the AGC on the ZN414.

Added a 10-turn pot on my vari-cap fine tuning. Lots easier to tune in SSB now.

I think I have to make a new pc board. That will be #3. After that, I will paint the enclosure (a Christmas cookie tin) and take some photos.

Rich

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Homebrew Radio ex-Silicon Valley

Thanks for the update Rich -- been following your work with great interest.

Do you think the radio is stable enough for CW on 40m? And, do you have any trouble with BCB breakthrough?

Really good stuff, thanks for sharing it.

73, Dan (WB3KBW)

Try this link. Direct, and less scary

That site has alot of neat stuff - thanks for posting it.

(and wow is Google translate getting good...)

Yes, it's very stable. I have a 7806 voltage regulator for the RF functions (not the audio amp.). Trouble with BC break-thru until I put a shielded cable on the BFO-to-IF line. KSRO 1350 KHz. will come in loud and clear if I try to touch the ZN414.

I can tune in an SSB signal on 40 mtr., turn off the power supply and then turn it on again without any drift.

I do notice that the wiring between the LO and the bandswitch on the front panel is very touchy. Just a slight movement will shift the tuning. That's going to be a long-term problem unless I can either shorten the runs or mechanically fix it in place. I can see now why most of the kit radios from Ramsey and TenTec have all the controls on the pc board.

Rich

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Homebrew Radio ex-Silicon Valley